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Retaining Wall Calculator · https://calcnaut.com/retaining-wall-calculator/

Retaining Wall Calculator

Enter your wall length and height, plus your block face size, to get the number of segmental retaining wall blocks you need, the courses, and the cap row. A waste allowance is built in, and the formula is shown.

Inputs

Enter your measurements

Wall length

ft / in

Wall height

ft / in

Block face length

Block face height

Base trench width

Enter the width required by the selected system. The cited Allan Block residential installation specifies a 24 in base trench; it is not derived from block depth.

Compacted base depth

Enter the compacted base depth from the selected system or approved plan. Do not infer it from wall height in this takeoff.

Drainage gravel width

Width of clean wall rock behind the block. Allan Block specifies at least 12 in.

Drainage gravel waste

Extra drainage stone for an irregular excavation and handling. Set 0 if your supplier allowance already includes it.

Waste allowance

User-selected extra for cuts at corners, curves and ends. Set it from the layout or supplier advice.

Price per block (optional)

Optional. Enter the price of one wall block to estimate the block cost.

This is an estimate, not professional advice. Check your inputs and verify the result against your plans and local building code before you build or order. See terms and disclaimer.

How this calculator works

Wall area (ft²) = Length(ft) × Height(ft)Blocks per ft² = 144 ÷ (Block length(in) × Block height(in))Grid blocks = ceil(Length(in) ÷ Block length) × ceil(Height(in) ÷ Block height)Wall blocks = ceil(Grid blocks × (1 + Waste% ÷ 100))Courses = Height(in) ÷ block height Cap row = blocks per courseBase pad (ft³) = (entered base width ÷ 12) × (entered base depth ÷ 12) × LengthDrainage gravel (ft³) = Wall face area × entered drainage width × (1 + drainage waste)

Enter your dimensions and the result updates instantly. A waste allowance is included so you order slightly over rather than running short.

Worked example

A 20 ft long by 3 ft high wall with 12 by 8 inch face blocks and a 10% waste allowance. Inputs: Wall length 20 ft, Wall height 3 ft, Block face length 12 in, Block face height 8 in, Waste allowance 10 %. Result: 110 .

Blocks for common retaining walls

Calculated for a 12 by 8 inch block face with a 10% waste allowance. Tap a size to load it above.

Wall size	Wall blocks	Courses (rows high)
20 ft × 2 ft	66	3	Use →
20 ft × 3 ft	110	5	Use →
30 ft × 3 ft	165	5	Use →
40 ft × 3 ft	220	5	Use →
30 ft × 4 ft	198	6	Use →
50 ft × 4 ft	330	6	Use →

Method & assumptions

A segmental retaining wall is built in whole courses. We round the wall length up to whole blocks per course and the wall height up to whole courses, multiply those two integer counts, then apply the entered waste allowance and round up again. This prevents the order quantity from falling below the grid shown in the same result when both dimensions leave partial blocks.

The cap row equals one course. Face sizes vary between systems, so enter the actual dimensions from the selected block. The count does not design corners, bond, setback or special units; confirm those against the manufacturer's layout.

The base and drainage fields are visible because they depend on the selected system, site and approved section. The cited Allan Block residential instructions specify a 24 inch base trench and describe wall rock within and behind its units. Those dimensions apply to that system, not every segmental block. Enter the base width, base depth and drainage width from the applicable manufacturer detail or project drawing.

Pro tips and common mistakes

Use the system section. Copy base, drainage, embedment and reinforcement dimensions from the selected manufacturer detail or approved drawing.
Measure the actual face. Enter the installed face length and height of the exact block. Product names alone do not determine units per square foot.
Separate special units. This result counts rectangular field units and one cap row. Count corners, curves and proprietary transition units from the layout.

By Calcnaut

Formula and declared-case checks run during the build; sources are listed for readers to review. Read our method and editorial policy.

Frequently asked questions

How many blocks do I need for a retaining wall?

Round the blocks per course and number of courses up separately, multiply them, then add waste. A 20 by 3 foot wall with 12 by 8 inch blocks needs a 20 by 5 grid, then 110 blocks with 10% waste, plus the separate cap order.

How high can a retaining wall be without engineering?

This quantity calculator does not establish a design threshold. Use the adopted code, manufacturer system limits and a qualified design for the site conditions.

What goes behind a retaining wall?

Enter the drainage width shown by the selected system or project drawing. The cited Allan Block guide describes wall rock within and behind its units, but other systems can require a different section.

Do I need cap blocks?

This calculator shows one cap row as a separate count. Confirm the cap unit, fixing method and any transitions in the selected system details.

Sources and references

These references and structured source records support the method and material data used on this page.

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Wall blocks
110

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